University of Connecticut American Physiological Society Renal Physiology Refresher Course -The Kidney and Acid-Base Regulation- Bruce M. Koeppen, M.D., Ph.D. University of Connecticut School of Medicine

Outline Review renal acidification from the perspective of what should be taught to first year medical students Overview – role of the kidneys HCO3- reabsorption along the nephron Generation of “New HCO3- ” Regulation of renal acid excretion

Overview Acid/Alkali Intake + Production = Acid/Alkali Excretion Metabolic Production of Acid/Alkali Acid/Alkali Excretion Acid/Alkali Intake + Production = Acid/Alkali Excretion

Overview FoodAcid/Alkali Impact Fruit Alkali Vegetables Alkali Meat Acid Grains Acid Dairy Products Acid “Typical” American Diet results in Net Endogenous Acid Production (NEAP)

HA = NEAP = 1 mEq/kg/day O2 Insulin Fat & Carbohydrate H2O + CO2 O2 HA + NaHCO3 NaA + H2O + CO2 Protein Organic Anions (e.g., citrate) “New” HCO3 Not shown is daily loss of HCO3- in feces – same as adding acid to body NH4A + HA HA = NEAP = 1 mEq/kg/day

Role of the Kidney NAE = UNH4+ V + UTA V – UHCO3- V NEAP New HCO3- NAE UNH4+ V, rate of ammonia excretion; UTAV, rate of titratable acid excretion; UHCO3-V, rate of bicarbonate excretion NAE = UNH4+ V + UTA V – UHCO3- V For definitions, see “Notes”

HCO3-Reabsorption Fig. 36-1. Reprinted with permission by Mosby.

Proximal Tubule HCO3- Reabsorption (2/3) NHE-3 (1/3) NBCe1 Fig. 36-2. Reprinted with permission by Mosby.

Similar but Different Proximal TALH Apical Transporters Na-H antiporter NHE-3 NHE-3 H-ATPase Basolateral Transporters Na-HCO3-symporter NBCe1 NBCn1 Cl-HCO3-antiporter AE-2 K-HCO3- symporter Carbonic Anhydrase Intracellular II II Membrane Bound IV IV

Collecting Duct HCO3- Transport H+ Secreting Intercalated cell HCO3- Secreting Intercalated cell Fig. 36-3. Reprinted with permission by Mosby. Basolateral Cl--HCO3-antiporter: AE-1 Apical Cl--HCO3-antiporter: Pendrin

Generation of New HCO3- NEA = UNH4+ V + UTA V – UHCO3- V HCO3- Reabsorption NEA = UNH4+ V + UTA V Titratable Acid HPO4= + H+ H2PO4- Fig. 36-4. Reprinted with permission by Mosby.

Generation of New HCO3- Glutamine 2 HCO3- 2 NH4+ 2H+ + urea

Figure 38-5. Reprinted with permission by Mosby. RhGlycoproteins

RhGlycoproteins RhAG: RhBG: RhCG: Red bloods cells Transports NH3 or functions as NH4+-H+antiporter RhBG: Kidney, liver, GI Tract Basolateral membrane of distal nephron segments (IC > PC) Electroneurtal versus electrogenic RhCG: Kidney, CNS, liver, muscle, testes, GI tract Apical, basolateral and intracellular localization of distal nephron segments (IC > PC) Electroneutral versus electrogenic

RhGlycoproteins Metabolic acidosis: RhCG expression Does not alter expression of RhBG Increases expression of RhCG RhCG expression Translocation to apical membrane from intracellular pool Figure showing percentage of relative apical RhCG in intercalated and principal cells during acidosis vs. control. (Fig. 3C, from Weiner and Hamm. Ann. Rev. Physiol. 69: 317, 2007) Figure 3C. From David Weiner and L. Lee Hamm. Molecular Mechanisms of Renal Ammonia Transport. Ann. Rev. Physiol. 69: 317, 2007. Available by subscription at http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.physiol.69.040705.142215

Regulation Acidosis Alkalosis NEA = UNH4+ V + UTA V – UHCO3- V

Response to Acidosis Acute response Chronic response cell pH results in more favorable kinetics of transporters Allosteric activation of transporters (e.g., NHE-3) Insertion of transporters into membrane from intracellular stores (e.g., NHE-3 and H+-ATPAse) Chronic response Transcription and/or translation of transporter genes and/or mRNA

Hormonal Regulation Endothelin-1 Glucocorticoid Increased ET-1 expression by proximal tubule and endothelial cells with acidosis ETB activation leads to phosphorylation of NHE-3 and insertion into apical membrane (Ca++ and tyrosine kinase dependent) – also NBCe1 in basolateral membrane ET-1 may also be involved in distal nephron regulation Glucocorticoid Secretion stimulated by acidosis Increased transcription, translation and expression of NHE-3 and NBCe1 Stimulation of ammoniagenesis

Hormonal Regulation Parathyroid Hormone (PTH) Secretion stimulated by acidosis PTH inhibits proximal Pi reabsorption Pi delivered distally results in an increase in titratable acid excretion

Response to Acidosis NEA = UNH4+ V + UTA V – UHCO3- V H+ secretion Cellular Acidosis ET-1 Glucocorticoid H+ secretion Cellular Acidosis ET-1 Glucocorticoid Ammoniagenesis Cellular Acidosis Glucocorticoid New HCO3- Titratable Acid NH4+ Secretion (RhCG) Fig. 36-6. Reprinted with permission by Mosby. Pi Reabsorption PTH NEA = UNH4+ V + UTA V – UHCO3- V

Response to Acidosis Urinary Net Charge UNC = [Na+] + [K+] – [Cl-] UNC is negative if NH4+ is excreted

Alterations in Renal Acid Excretion Increased Acid Excretion ConditionSite ECF Volume Contraction A-II Proximal and Distal Aldosterone Distal and Collecting Duct Hypokalemia Proximal and IC’s Decreased Acid Excretion ECF Volume Expansion Proximal, Distal & Collecting Duct Hypoaldosteronism Distal and Collecting Duct Hyperkalemia Proximal AII, angiotensin II; ECF, extracellular fluid; IC, intercalated cell For definitions, see “Notes”

Summary Renal NAE must equal NEAP NAE = UNH4+ V + UTA V – UHCO3- V NAE is altered in response to acid-base disorders Alterations in numbers and activities of acid-base transporters (H+, HCO3-& NH4+) Mediators include: pHi, ET-1 &Glucocorticoid pHi, intracellular pH